In order to compare data-quality of different data-collection modes, multitrait-multimethod (MTMM) experiments have been implemented in a mixed-mode experiment parallel to the European Social Survey (ESS) fourth round (2008/2009). Special interest lies in measurement effects between the modes which refer to the pure impact of a data-collection mode on the quality. Nevertheless, mere comparison between quality estimates of the different modes does not allow drawing conclusions about measurement effects. Indeed, measurement effects may be completely confounded with selection effects which refer to differences in respondents compositions across the modes. However, by comparing the mixed-mode data with the main ESS data and treating the dataset of origin as an instrumental variable, some conditional measurement effects and selection effects can be disentangled. This paper provides a preliminary exploratory analysis of this approach. The results generally yield low to fair measurement effects while the selection effects on some items are rather large.